1. Field of the Invention
The present invention relates to a piezoelectric actuator, a method of manufacturing same, and a liquid ejection head, and more particularly to technologies that can reduce a thermal stress induced by film formation of a piezoelectric body.
2. Description of the Related Art
Conventionally, the principal method of manufacturing an inkjet head is a method which involves bonding a bulk polished piezoelectric body member (made of lead zirconate titanate (Pb(ZrTi)O3) (PZT), for example) and a calcined green sheet, to a diaphragm plate, by means of an adhesive. In this case, there is no need to take account of thermal stress in the piezoelectric body.
Furthermore, in a method of manufacturing inkjet head described in Japanese Patent Application Publication No. 2003-309299, a piezoelectric film is grown epitaxially on an epitaxial substrate, a silicon substrate is bonded directly to the opposite side of the piezoelectric film, and then the epitaxial substrate is separated from the piezoelectric film. In this case, the thermal stress induced on the boundary between the piezoelectric film and the epitaxial substrate by the formation of the piezoelectric film can be eliminated by removing the epitaxial substrate.
Moreover, recently, it is necessary to form the piezoelectric body as a thin film in accordance with increasing density in inkjet heads. Therefore, there are various examples that a piezoelectric body is formed on a diaphragm plate by the sputtering method.
On the other hand, recently, in the field of micro electrical mechanical systems (MEMS), technologies are suggested that elements such as sensors and actuators are integrated in yet higher levels by using piezoelectric ceramic, and are forming films on those elements in order to achieve practical use. An aerosol deposition method receives attention in those technologies, which is known as a film formation technique for use with ceramics, metals, and the like.
The aerosol deposition method is a method in which an aerosol is generated from a powder of raw material, the aerosol is sprayed onto a substrate, and a film is formed by deposition of the powdered material due to its impact energy. It is also called a “spray volume method” or a “gas deposition volume method”.
When manufacturing a liquid ejection head, such as an inkjet head, it is proposed that a piezoelectric body and the like may be formed by the aerosol deposition method described above (see Japanese Patent Application Publication No. 2003-136714).
The film thickness of a piezoelectric body formed by the sputtering method on a diaphragm is approximately 3 μm. Therefore, since the film is thin, it does not give rise to notable deformation of the diaphragm even if there is stress in the film. Hence, no particular importance is attached to controlling stress in the film.
On the other hand, if a piezoelectric body or the like is formed by the aerosol deposition method, as described in Japanese Patent Application Publication No. 2003-136714, a strong compressive stress acts in the aerosol deposition film which has a fine structure, and the temperature during aerosol deposition film formation is approximately 600° C. Therefore, thermal stress acts on the film after film formation due to the difference between thermal expansion coefficients of the piezoelectric body and the diaphragm, and in particular, when the film is formed on a thin diaphragm having a thickness of 30 μm or less, then the diaphragm deforms due to the stress. Furthermore, if the film thickness of the piezoelectric body is 1 μm or more, and more particularly, approximately 10 μm, then the deformation due to stress in the film occurs remarkably.
FIG. 13A shows an ideal state in which there is no film stress in a piezoelectric body 2 formed on a diaphragm 1, so that there is no deformation of the diaphragm 1. FIG. 13B shows a state where a compressive stress acts on the piezoelectric body 2, so that the diaphragm 1 deforms unevenly due to the stress.
When stress deformation occurs in the diaphragm 1 as shown in FIG. 13B, there are following problems.
(1) Since a gap arises when the diaphragm 1 is bonded to the ink chamber partitions 3, the following problems arise, consequently.                a) Since the ink chambers 4 cannot be divided by the ink chamber partitions 3, then there is movement of ink between adjacently positioned ink chambers 4, and ink may gather in the gaps, thereby making it impossible to control the volume of ink that is ejected from the nozzles.        b) Since regions where the diaphragm 1 is not fixed to the ink chamber partitions 3 arise, then variations occur in the displacement volume and torque, and hence image defects arise due to non-uniformities in ink ejection throughout the surface.        c) Since the diaphragm 1 and the ink chamber partitions 3 can not be reliably fixed by bonding, then long-term durability cannot be guaranteed.        
(2) Since deformation occurs in the diaphragm 1, then the volume of the ink chamber 4 varies, and variation may arise in the ejected ink volume even if the diaphragm 1 is driven at the same displacement volume, whereby image defects may occur due to non-uniformities in ink ejection throughout the surface.
(3) When deformation occurs in the diaphragm 1, sufficient alignment accuracy is not obtained in the subsequent process of wiring to the electrodes for driving the piezoelectric bodies 2, whereby wiring defects arise.